A signal is a kind of "message" that a program can send to another program; an example can be the hit of the CRTL + C keys combination, which produces an INT signal, which usually stops the script execution. "Masking" a signal in a script means associating to this signal a function to be executed when the signal is received.
In the bash the system signals can be masked through the trap signal, according to the following syntax:

trap function_name signal_name

Here there's an example:

#!/bin/bash

# The following script shows the usage of the trap function. # In the function three countdowns from 10 seconds are present.# You can try to stop the countdowns by pressing CRTL-C # (INT signal) and verifying the script behaviour.# During the first countdown, the INT signal is masked by an # error function: the arrival of this signal causes the script # to display this error message. # Then during the second countdown the signal INT is disabled:# its reception causes no effect.# Finally durign the third countdown the mask is removed: # it is now possible to stop the count.

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